The surface of powder or grain such as flour, rice flour, spices such as pepper, tea powder, chlorella powder or grain, and cosmetic powder is contaminated with microorganisms such as airborne bacteria and fungi. Proliferation of the microorganisms degrades the quality of the powder or grain with time. When conditions are suitable for the proliferation of the microorganisms, for example, high temperatures and humidity, especially during the transportation or processing of the powder or grain, the microorganisms may proliferate explosively to cause great damage.
Furthermore, powder or grain may be contaminated with insect pests such as maize weevil and Indian meal moth or with their eggs to cause the quality degradation of the powder or grain.
Various sterilization methods have been studied for preventing the proliferation of such harmful organisms as microorganisms and insect pests and been put into practical use. For example, as for the sterilization of microorganisms, powder or grain commonly undergoes sterilization methods in which the powder or grain is sterilized (thermally sterilized) by indirect or direct heating with Joule heating, induction heating, heated air, hot water, steam, superheated steam, or pressurized steam.
In order to sterilize microorganisms by heating, microorganisms are exposed to heating at a predetermined temperature for a predetermined period for sterilization. A large wealth of knowledge has been accumulated about the relation between the predetermined temperature and the predetermined period for heat exposure. Thermal sterilization is widely used in the food industry because thermal sterilization is a highly safe method for sterilizing food and has been proven in various sterilization applications to accumulate the knowledge and data for securing a predetermined sterilization level.
For example, Patent. Document 1 discloses in paragraph [0009] an apparatus that includes a raw material supplying unit (reference numerals 2 to 8 in FIG. 1) in which powder or grain is supplied with pressure with air that is heated and pressurized at a temperature of about 80 to 200° C. and a pressure of ambient pressure to about 10 kg/cm2G (corresponding to 0.1 to 1 MPaG) toward a first nozzle and a steam supplying unit (reference numerals 14 to 17 in FIG. 1) in which steam 13 and air 14 are mixed to be supplied to a first throttle nozzle 9. It is supposed that the powder or grain supplied from the raw material supplying unit and the mixed gas of steam and air supplied from the steam supplying unit are mixed in a heating apparatus 12 to perform thermal sterilization. It is described that the heating apparatus 12 has an outlet provided with a second throttle nozzle (paragraph [0010]).
The invention of Patent Document 1 superficially resembles an embodiment of the apparatus of the present invention. However, paragraph [0009] describes that the temperature is 80 to 200° C., the pressure is ambient pressure to 10 kg/cm2G, and the residence time is 3 to 60 seconds in the heating apparatus 12. As described in paragraph [0034], when the heating apparatus is a straight pipe, the flow rate of the mixed gas is 20 m/second, and the residence time is 0.5 to 2 seconds, the straight pipe has a large length of 10 to 40 m. Accordingly, a cyclone as shown in FIG. 8 is required to gain the residence time and to downsize the apparatus.
This is backed by the fact that, in an embodiment described in paragraph [0040] of Patent Document 1, the flow rate of the mixed gas is 15 m/second and the residence time is 4 seconds, and when a straight pipe is used, its length becomes as large as 60 m. This is supposed to be because the sterilization method of Patent Document 1 mainly depends on heating and a heat history sufficient for killing microorganisms cannot be obtained unless heating lasts for at least several seconds.
Patent Document 2 provides a heating method that includes supplying powder or grain material into a pressurized and heated medium flow such as superheated steam to mix and transfer them, transferring the transferring medium flow of the pressurized and heated medium mixed with the powder or grain material into a heated pipe generating a swirling flow provided downstream, and swirling the transferring medium along the flow in the heated pipe generating a swirling flow to spirally transfer the powder or grain material. The heated pipe generating a swirling flow is heated indirectly. As described in column 4, the method is intended to thermally sterilize and thermally denature powder or grain.
Patent Document 2 describes in column 6 that the condition of direct heating is preferably at a comparatively low temperature for sterilization, and the raw material is treated for 0.1 to 3 seconds by direct contact with saturated steam at a gauge pressure of 5 kg/cm2 or less and preferably of 0.5 to 2.5 kg/cm2, or with superheated steam at a gauge pressure of 4 kg/cm2 or less and a temperature of 300° C. or less and preferably a pressure of 0.1 to 3 kg/cm2 and a temperature of 250° C. or less. However, even the shortest treatment time among the embodiments requires 0.7 second, using superheated steam at 194° C. (Embodiment 1). In comparison with Patent Document 1, the treatment temperature is higher, but the treatment time is considerably reduced. However, there is no description whether the bacterium used in the embodiments is a heat-resistant bacterium or not. Treatment of heat-resistant bacteria may require a longer time Thus, further reduction in the treatment time and treatment temperature may reduce the quality degradation of the sterilized material.
Furthermore, Patent Document 2 describes in column 4 that pressure control provides smooth and efficient thermal denaturation, and in column 7 that, when a nozzle is used as the discharging device, pressure is reduced in a shorter period than with a rotary valve to obtain larger swelling. However, there is no description that this swelling (pressure reduction) contributes to sterilization. Thus, the method is achieved by thermal sterilization. Cited Document 2 specifically describes that provided is a heating method and a heating apparatus by which powder or grain material is efficiently thermally sterilized with a heated medium of superheated steam and by which powder or grain materials that are cereals, food, and the like are efficiently thermally denatured as well as the apparatus or system can be downsized (line 5 in column 4). Furthermore, it describes that then the raw material flowing in the pipe as a swirling flow flows along the pipe wall to be heated because the pipe is indirectly heated with the heating unit, and thus the raw material is efficiently heated, and that then the raw material is sterilized or thermally denatured because the transferring stream is pressurized, and the pressure of the stream is controlled with the downstream throttle depending on the progress of heating (line 10 in column 5). The description clearly shows that Cited Document 2 discloses sterilization involving heating.
Patent Document 3 provides a method for sterilizing powder or grain that includes aspirating powder or grain by an ejector 3 (FIG. 1) using superheated steam as a drive source, compressing and mixing the powder or grain and the superheated steam, thermally sterilizing the powder or grain, and thereafter separating the powder or grain from the superheated steam to collect, and in which both of the powder or grain and the superheated steam are aspirated into the ejector 3 using the superheated steam as the drive source. It describes in paragraph [0016] that the material and the superheated steam are aspirated, then compressed,mixed, and heated in a diffuser 19 accompanied with superheated steam that is discharged from the outlet of a nozzle 17, and that thermal conductivity in this process is large and thus the material is thermally sterilized rapidly. The description shows that Cited Document 3 also discloses sterilization by the latent heat of steam.
Each of the techniques of Patent Documents 1 to 3 is a thermal sterilization method in which microorganisms such as bacteria and fungi adhering to powder or grain raw materials are heated to raise the temperature of the whole of the powder or grain, and thus polysaccharides, proteins, lipids, nucleic acids, and the like included in the microorganisms are denatured.
However, such methods have a problem that, in order to keep the level sufficient for sterilization by such conventional heating method, powder or grain raw materials themselves are heated unnecessarily. Then, part of the starch, protein, lipid, and the like in the raw materials is denatured to change their characteristics as food raw materials and the like, and thus their commercial value is reduced. Ideally, such a short heating time that only microorganisms present on the surface of powder or grain are heated while the inside of the powder or grain remains unheated is desirable. However, because such a short heating time cannot provide a long enough heat history to kill the microorganisms, the heating time cannot be reduced.
Accordingly, in the conventional thermal sterilization methods, a sterilization condition under which the heat history of the powder or grain raw material becomes as small as possible is identified and the heating condition is controlled depending on the purpose of sterilization. In other words, even when the sterilization level is intended to be high, because heat degrades the powder or grain raw material or heating in the presence of oxygen causes rapid oxidation to significantly degrade the quality of the material, the conventional thermal sterilization methods have limitations in increasing the sterilization level.
In particular, microorganisms forming spores (heat-resistant bacteria) are covered with robust superficial tissues and thus can be killed only in an extremely stronger sterilization condition in comparison with microorganisms of normal vegetative cells. Thus, when reliable sterilization is required, the heating time must be set long.
Therefore, with respect to the heat-resistant bacteria, non-thermal sterilization by radiation exposure, ultraviolet exposure, ozone, or the like has been studied and some of them are put into practical use. In these methods, radiation rays or ultraviolet rays are applied and thus the energy of electromagnetic waves thereof destroys tissues and the like for sterilization. On the other hand, ozone has strong oxidative effect to destroy bacterium tissues for sterilization. However, these non-thermal sterilization methods have problems that they have less certainty of sterilization and a part not irradiated with the electromagnetic waves is not sterilized. In the method employing ozone, when microorganisms have a part that cannot be in contact with ozone, the part is not sterilized. Furthermore, radiation exposure has a safety concern especially when applied to food, and thus the sterilization methods are not permitted in Japan and other countries.
In contrast, examples of the method for killing insects and eggs include a method for killing insects and eggs by reduced pressure as in Patent Document 4. According to an embodiment, it is described that the method includes placing a material to be treated in a closed container under a pressure of 5 to 60 atmospheres for about 3 to 20 minutes, and rapidly or slowly reducing the pressure from the above-described condition to kill insects. The method must be a batch-wise method in order to keep the pressurized condition for several minutes, and thus has a problem of low treatment efficiency. The method has another problem that it requires an apparatus that can withstand a pressure of 60 atmospheres as in Embodiment 4 and thus the apparatus size increases.
Patent Document 1: Japanese Patent Application Publication No. 2000-24091 (FIG. 1, paragraphs [0009], [0010], and [0040])
Patent Document 2: Japanese Examined Patent Application Publication No. 5-53, (claims 1, 4, 5, for example)
Patent Document 3: Japanese Patent Application Publication No. 2000-157615 (paragraph [0016])
Patent Document 4: Japanese Examined Patent Application Publication No. 7-114674